Observations of long-period (10 to 100 sec) seismic noise at several worldwide locations

This paper describes an investigation of the spectral shape and the level of seismic background noise in the period range 10–100 sec recorded at high-gain seismograph stations in six different regions of the world. Both vertical and horizontal power spectra were computed. These spectra are based on data recorded digitally at each site by equipment having a dynamic range of about 70 db. Each spectrum is characterized by a pronounced minimum of the noise level between 30 and 40 sec. The absolute values of this noise minimum are found to be approximately the same at these six sites, which differ greatly in both geographic location and depth of overburden. This noise minimum, or window, which appears to be world-wide in extent and extremely stable both spatially and temporally, is of great importance to the discrimination of underground explosions from earthquakes.